(1) Comparative studies on the replication of an alphavirus (Sindbis) in cultured mosquito cells and vertebrate cells have been facilitated by the following observations: (a) Clones of A. albopictus cells which are, in contrast to the "wild-type" cells, susceptible to CPE support increased viral RNA synthesis which is further promoted by increased temperature; (b) ouabain, an inhibitor of plasma membrane Na ion/K ion AtPase, reduces SV replication and RNA synthesis in ouabain-sensitive cells but not cloned ouabain-resistant A. albopictus cells; (c) virazole, an inhibitor of GMP synthesis, reduces SV replication in CPE-susceptible A. albopictus cells but not in the WT mosquito or in vertebrate cells. The mechanisms underlying these effects will be studied. (2) The characteristics of SV generated in whole mosquitoes persistently infected by intrathoracic inoculation are being studied, specifically with regard to temperature sensitivity, defectiveness, interfering capacity, and RNA patterns. (3) Studies on flaviviruses (dengue-2) concern characterization of intermediates in viral RNA replication, structural features of virion and intermediate RNA species, transcriptional and translational events, as well as continued efforts to characterize the virion-like particles which accumulate in dengue-2 infected Vero cells.